This invention relates generally to detecting low refrigerant charge in refrigeration systems, particularly automotive air conditioning systems, and more particularly concerns an apparatus which uses a differential pressure sensor to detect high superheat at the evaporator exit. As used herein, the term "refrigeration system" refers to refrigerators, air conditioners or any other system which produces a refrigeration effect. However, the present invention is most applicable to air conditioning systems for automobiles.
In typical automotive air conditioning systems, refrigerant tends to leak through hose permeation and the rotating compressor shaft seal. Refrigerant leakage causes many problems. First, the release of refrigerants into the environment is believed to cause environmental damage. For this reason, enactment of a federal environmental protection law requiring low charge detection systems for automobiles is anticipated. Second, when the refrigerant charge becomes insufficient, the reliability and cooling performance of the system suffer. Thus, a refrigeration system with a low charge is inefficient and wasteful of energy. Furthermore, low charge causes increased compressor operating temperatures while lowering mass flow rate. Reduced mass flow rate can result in insufficient flow of lubricating oil to the compressor. The lack of oil combined with high temperatures eventually causes compressor failure. Thus, there is much interest in developing means for detecting low charges.
There are a number of known ways to detect low charge, but many of these present certain disadvantages. For instance, determining low charge by measuring refrigerant inventory will not always be accurate because the optimum inventory level varies in accordance with many variables such as compressor speed, ambient and interior temperatures, blower speed, and component volumes. Other systems employ a low pressure cut-off switch which deactivates the compressor when system pressure falls below a predetermined threshold. However, since system pressures fluctuate greatly during proper operation (25-46 psi is typical), the predetermined threshold pressure must be set very low, such as 10-20 psi. As a result, the low pressure cut-off switch is effective to indicate only a severe loss of charge resulting from a ruptured hose or complete shaft seal failure. This switch will not detect marginal low charge conditions.
One way of detecting low charge which avoids the above problems is to monitor the superheat at the evaporator exit. Superheat is the amount of temperature above the saturation temperature of the refrigerant. It is well known that excessive evaporator exit superheat is indicative of insufficient charge. For instance, U.S. Pat. No. 4,677,830 to Seiji Sumikawa et al. discloses providing a pressure sensor and a temperature sensor near the evaporator exit. An electronic control unit converts the measured pressure to a corresponding saturation temperature of the refrigerant. The difference between the measured temperature and the corresponding saturation temperature (i.e., the superheat) is then compared to a predetermined reference value to determine whether there is a sufficient quantity of refrigerant. If an insufficient charge is detected, the compressor is rendered inoperative by a signal from the electronic control unit.
U.S. Pat. No. 4,745,765 to Edward D. Pettitt discloses another device which uses evaporator exit superheat to detect low charge. This device operates on the pressure differential between the refrigerant pressure at the evaporator exit and a reference pressure. The device includes a pair of corrugated metal bellows members; one sealed with a certain amount of refrigerant, the other being open to the evaporator exit. An arm member carrying a switch contact is mounted between the two bellows members for movement therewith. As the amount of superheat rises, the reference pressure exceeds the evaporator exit pressure. This causes the sealed bellows member to expand and move the arm member. At some point, the arm member will be moved far enough so that the switch contact engages a second contact mounted on a bimetallic disk. The engagement of the contacts closes a circuit which either provides an alert signal or shuts down the system.
The dual bellows members of U.S. Pat. No. 4,745,765 present a mechanically complicated arrangement. The many moving parts and the attendant difficulty of construction produces high costs. The bellows are also subjected to repeated expansion and contraction causing rapid wear and a short service life. Furthermore, the bellows arrangement can tend to give indefinite signals because the contacts are not necessarily positively forced into contact. That is, the bellows members may assume a position where the contacts are only lightly touching. In this case, the contacts would tend to fluctuate in and out of contact with each other, thereby producing an indefinite signal.